.\" -*- nroff -*-
.\" Copyright 2013 Los Alamos National Security, LLC. All rights reserved.
.\" Copyright 2010 Cisco Systems, Inc.  All rights reserved.
.\" Copyright 2006-2008 Sun Microsystems, Inc.
.\" Copyright (c) 1996 Thinking Machines Corporation
.\" Copyright (c) 2020      Google, LLC. All rights reserved.
.\" $COPYRIGHT$
.TH MPI_Comm_create 3 "#OMPI_DATE#" "#PACKAGE_VERSION#" "#PACKAGE_NAME#"
.SH NAME
\fBMPI_Comm_create\fP \- Creates a new communicator.

.SH SYNTAX
.ft R
.SH C Syntax
.nf
#include <mpi.h>
int MPI_Comm_create(MPI_Comm \fIcomm\fP, MPI_Group\fI group\fP, MPI_Comm\fI *newcomm\fP)

.fi
.SH Fortran Syntax
.nf
USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_COMM_CREATE(\fICOMM, GROUP, NEWCOMM, IERROR\fP)
	INTEGER	\fICOMM, GROUP, NEWCOMM, IERROR\fP

.fi
.SH Fortran 2008 Syntax
.nf
USE mpi_f08
MPI_Comm_create(\fIcomm\fP, \fIgroup\fP, \fInewcomm\fP, \fIierror\fP)
	TYPE(MPI_Comm), INTENT(IN) :: \fIcomm\fP
	TYPE(MPI_Group), INTENT(IN) :: \fIgroup\fP
	TYPE(MPI_Comm), INTENT(OUT) :: \fInewcomm\fP
	INTEGER, OPTIONAL, INTENT(OUT) :: \fIierror\fP

.fi
.SH INPUT PARAMETER
.ft R
.TP 1i
comm
Communicator (handle).
.TP 1i
group
Group, which is a subset of the group of comm (handle).

.SH OUTPUT PARAMETERS
.ft R
.TP 1i
newcomm
New communicator (handle).
.ft R
.TP 1i
IERROR
Fortran only: Error status (integer).

.SH DESCRIPTION
.ft R
This function creates a new communicator newcomm with communication
group defined by group and a new context. The function sets
\fInewcomm\fR to a new communicator that spans all the processes that
are in the group.  It sets \fInewcomm\fR to MPI_COMM_NULL for
processes that are not in the group.

Each process must call with a \fIgroup\fR argument that is a subgroup
of the group associated with \fIcomm\fR; this could be
MPI_GROUP_EMPTY. The processes may specify different values for the
\fIgroup\fR argument. If a process calls with a non-empty \fIgroup\fR,
then all processes in that group must call the function with the same
\fIgroup\fR as argument, that is: the same processes in the same
order. Otherwise the call is erroneous.
.sp
.LP

.SH NOTES
MPI_Comm_create provides a means of making a subset of processes for the purpose of separate MIMD computation, with separate communication space. \fInewcomm\fR, which is created by MPI_Comm_create, can be used in subsequent calls to MPI_Comm_create (or other communicator constructors) to further subdivide a computation into parallel sub-computations. A more general service is provided by MPI_Comm_split.

.SH ERRORS
Almost all MPI routines return an error value; C routines as the value of the function and Fortran routines in the last argument.
.sp
Before the error value is returned, the current MPI error handler is
called. By default, this error handler aborts the MPI job, except for I/O function errors. The error handler may be changed with MPI_Comm_set_errhandler; the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarantee that an MPI program can continue past an error.

.SH SEE ALSO
.ft R
.sp
MPI_Comm_split
.sp
MPI_Intercomm_create
MPI_Comm_create_group
